A bomb shelter is a place for people to take refuge in during a bombing. There are many bomb shelters in much of Europe, dating back to World War II or the Cold War. With the end of the Cold War, many of these are in disrepair. Some of the largest were segments of the Underground in London which were converted into huge air raid shelters that could hold 170,000 people or more. To cope with the challenge of disease, chemical toilets were installed, and reinforced floodgates were available at many shelters, to protect them from flooding in case of burst river embankments due to bomb damage.
Though the historic bomb shelter had its primary role during WWII, since the 1950s many more bomb shelters have been created to cope with the risk of nuclear war. These special shelters are usually fallout shelters as well as bomb shelters, capable of protecting the occupants from radioactive particles which would descend from the sky in the event of ground-burst atomic bombing. Contrary to popular belief, filtering the air in such a bomb shelter would not be necessary, as the most radioactive particles would be too large to be suspended in the air. In any case, a great risk to life and limb is likely to come from poor ventilation or contaminated food and water rather than contaminated air.
Several countries around the world, especially in Europe, have made it national policy to construct a bomb shelter for every community. The leader is Switzerland, which has the highest shelter-to-person ratio of any nation, enough to provide shelter for its entire population for up to two years in the aftermath of a nuclear war. In Finland, every structure larger than 600 m2 must have a bomb shelter, and in Sweden, every structure larger than 1000 m2 must have one. The essential feature of any fallout shelter is enough shielding to protect its inhabitants from radiation emitting from any "hot" fallout particles on the ground outside.
There are several myths about bomb and fallout shelters that would put the public at greater risk during a nuclear war. The first is that only an expensive, concrete, purpose-built shelter can protect a family during a nuclear war. This is untrue -- even a hand-dug trench in a backyard can provide proper protection against fallout as long as it is covered in three feet of dirt and has entrances at ninety-degree angles to the main trench. Gamma rays from fallout only travel in straight lines, so as long as the occupants are sufficiently shielded from fresh fallout, they should survive. Another myth is that it would take years or centuries for fallout radiation to dissipate. In actuality, a couple of weeks would likely be sufficient in most cases, and a couple of months in only the very worst cases.